1. Field of the Invention
The present invention relates to a piezoelectric thin-film resonator used for filters in a high-frequency band (RF band, particularly, GHz band or more) of communication apparatuses, such as mobile phones. The present invention also relates to a piezoelectric filter including the resonator and an electronic component including the piezoelectric filter, such as a duplexer.
2. Description of the Related Art
In recent years, filters used in a high-frequency band (RF band, particularly, GHz band or more) for use in communication apparatuses, such as mobile phones, have been developed by using piezoelectric resonators that are small, lightweight, and highly resistant to vibration and shock. Furthermore, variations in products are small, highly reliable, and circuits need not be adjusted. Thus, mounting can be automated and simplified. In addition, a piezoelectric resonator having a high frequency can be easily manufactured.
The above-described piezoelectric resonator includes a piezoelectric substrate and electrodes provided on both principal surfaces thereof. The piezoelectric resonator operates in a thickness-longitudinal vibration mode or a thickness-shear vibration mode. The resonance frequency of the piezoelectric resonator in the thickness-longitudinal vibration mode of the piezoelectric substrate is inversely proportional to the thickness of the piezoelectric substrate, and thus the piezoelectric substrate must be extremely thin for a use in an ultra high-frequency region.
However, the thickness of the piezoelectric substrate can be reduced only within limits of mechanical strength and handling, and a practical high-frequency limit is several hundred MHz in a fundamental mode. In order to overcome such a problem, the usage of a piezoelectric thin-film resonator has been proposed for use in filters and resonators (for example, reference 1: Japanese Unexamined Patent Application Publication No. 2001-168674, published on Jun. 22, 2001).
In this piezoelectric thin-film resonator, a thin-film supporting portion can be thinned by using micromachining techniques, and a thin piezoelectric thin-film can be formed by sputtering or the like. Thus, a high-frequency characteristic of several hundred to several thousand MHz can be achieved.
Also, a piezoelectric resonator which includes an SiO2 thin-film having a positive resonance-frequency temperature characteristic has been proposed so as to improve the temperature characteristic of the resonance frequency (reference 2: Japanese Unexamined Patent Application Publication No. 58-121817, published on Jul. 20, 1983 and reference 3: Japanese Unexamined Patent Application Publication No. 58-137317, published on Aug. 15, 1983).
Also, a piezoelectric resonator including a lower electrode, a piezoelectric layer including AlN, and an upper electrode has been proposed in order to increase Q of the piezoelectric resonator (reference 4: Japanese Unexamined Patent Application Publication No. 2000-69594, published on Mar. 3, 2000). In this piezoelectric resonator, the electrodes include Mo, which has a low thermoelesticity loss.
Further, in a piezoelectric resonator shown in reference 5 (Japanese Unexamined Patent Application Publication No. 2001-156582, published on Jun. 8, 2001), an electrode includes two layers of Pt and Al in order to reduce the ratio of Pt in the electrode. Also, Al having a low resistivity is used. With this configuration, by reducing the ratio of Pt in the electrode in order to reduce the mass additional effect and by increasing the ratio of Al having a low resistivity in order to reduce the resistance of the entire electrode, the resonance frequency can be improved and the Q of the piezoelectric resonator can be increased.
However, in the piezoelectric resonator according to reference 5, the filter characteristics widely vary and high manufacturing precision is required, which leads to an increase in the manufacturing cost.
Also, as in references 2 and 3, when a SiO2 thin-film is used to improve the temperature characteristic, Q of the piezoelectric resonator decreases because the stiffness of SiO2 is low.
On the other hand, because the piezoelectric resonator shown in reference 4 includes Mo and AlN, the temperature coefficient of frequency (TFC) is unfavorable and the TFC cannot be adjusted.
U.S. Pat. No. 6,323,744 (publication date: Nov. 27, 2001) discloses a filter circuit using a piezoelectric resonator. The filter circuit is a ladder filter having parallel resonators and series resonators. Each of the parallel resonators has a top ground (GND) electrode, a lower electrode, and a path for connecting the top GND electrode to an individual external GND.
Moreover, Japanese Unexamined Patent Application Publication No. 2000-269780 (publication date: Sep. 29, 2000) discloses a balanced multiple-mode piezoelectric filter having four electrodes arranged in a 2 by 2 matrix. In the filter, two of the electrodes which are aligned in the same column are paired and one electrode of the pair functions as an input side and the other electrode functions as an output side.
The filter disclosed in U.S. Pat. No. 6,323,744 has a problem in that, because input and output terminals are unbalanced terminals, the filter is not applicable to an electronic component, such as an integrated circuit, having balanced input and output terminals, which are becoming the mainstream.
In the filter disclosed in Japanese Unexamined Patent Application Publication No. 2000-269780, two standing waves, a first symmetric resonant mode and a first anti-symmetric resonant mode, are acoustically coupled. Electric signals input from an input end are acoustically transformed, propagate to an output end, and are transformed into electric signals at the output end. Therefore, acoustic transformation is performed twice, so that transformation energy loss is increased and insertion loss is thus increased.